Failure diagnosis device of emission control system

Abstract

In a failure diagnosis device of an emission control system that utilizes an electrode-based PM sensor provided downstream of a particulate filter in an exhaust conduit to diagnose a failure of the particulate filter, disclosed embodiments may suppress reduction of accuracy of diagnosis of a failure due to in-cylinder rich control. The failure diagnosis device of the emission control system starts application of a predetermined voltage to electrodes of the electrode-based PM sensor after a sensor recovery process that removes PM depositing between the electrodes of the PM sensor, and diagnoses a failure of the particulate filter based on an output of the PM sensor measured after elapse of a predetermined time period since the start of application of the predetermined voltage. The failure diagnosis device performs the sensor recovery process during in-cylinder rich control or triggered by termination of the in-cylinder rich control and subsequently performs a measurement process after termination of the in-cylinder rich control and the sensor recovery process.

What is claimed is: 
     
       1. A failure diagnosis device for an emission control system, wherein the emission control system includes a particulate filter that is placed in an exhaust conduit of an internal combustion engine and that is configured to trap PM in exhaust gas; an exhaust gas purification device that is placed upstream of the particulate filter in the exhaust conduit and that is configured to purify the exhaust gas by utilizing a non-combusted fuel component included in the exhaust gas; and a supplier that is configured to perform in-cylinder rich control of changing an air-fuel ratio of an air-fuel mixture subjected to combustion in the internal combustion engine to a rich air-fuel ratio which is lower than a stoichiometric air-fuel ratio, so as to supply the non-combusted fuel component to the exhaust gas purification device,
 the failure diagnosis device comprising: 
 a PM sensor that is provided to detect an amount of PM flowing out of the particulate filter, the PM sensor having a sensor element that includes electrodes opposed to each other across an insulating layer and a heater that is configured to heat the sensor element, the PM sensor being configured to output an electric signal relating to an amount of PM depositing between the electrodes under application of a predetermined voltage to the sensor element; and 
 a controller comprising at least one processor configured to perform a process of diagnosing a failure of the particulate filter, based on an output value of the PM sensor, wherein 
 the controller is programmed to:
 perform a sensor recovery process that controls the heater to heat the sensor element to a temperature that allows for oxidation of PM and thereby oxidizes and removes the PM depositing between the electrodes, and a measurement process that starts application of the predetermined voltage to the sensor element after termination of the sensor recovery process and measures an output value of the PM sensor when a predetermined time period has elapsed since start of application of the predetermined voltage; and 
 diagnose a failure of the particulate filter by comparing the obtained output value of the PM sensor with a predefined reference value, 
 wherein the controller performs the sensor recovery process during the in-cylinder rich control or triggered by termination of the in-cylinder rich control and subsequently performs the measurement process after termination of the in-cylinder rich control and the sensor recovery process. 
 
 
     
     
       2. The failure diagnosis device of the emission control system according to  claim 1 , wherein
 when the sensor recovery process is triggered by termination of the in-cylinder rich control, the controller performs preheat treatment that controls the heater to heat the sensor element to a specified temperature lower than the temperature that allows for oxidation of PM during the in-cylinder rich control. 
 
     
     
       3. The failure diagnosis device of the emission control system according to  claim 1 , wherein the controller is further programmed to:
 predict whether the in-cylinder rich control is performed in the predetermined time period, before the sensor recovery process is actually performed, wherein 
 when it is predicted that the in-cylinder rich control is performed in the predetermined time period, the controller performs the sensor recovery process during the in-cylinder rich control or triggered by termination of the in-cylinder rich control and subsequently performs the measurement process after termination of the in-cylinder rich control and the sensor recovery process, and 
 when it is predicted that the in-cylinder rich control is not performed in the predetermined time period, the controller performs the sensor recovery process and subsequently performs the measurement process after termination of the sensor recovery process. 
 
     
     
       4. The failure diagnosis device of the emission control system according to  claim 2 , wherein the controller is further programmed to:
 predict whether the in-cylinder rich control is performed in the predetermined time period, before the sensor recovery process is actually performed, wherein 
 when it is predicted that the in-cylinder rich control is performed in the predetermined time period, the controller is programmed to perform the sensor recovery process during the in-cylinder rich control or triggered by termination of the in-cylinder rich control and subsequently performs the measurement process after termination of the in-cylinder rich control and the sensor recovery process, and 
 when it is predicted that the in-cylinder rich control is not performed in the predetermined time period, the controller is programmed to perform the sensor recovery process and subsequently performs the measurement process after termination of the sensor recovery process. 
 
     
     
       5. The failure diagnosis device of the emission control system according to  claim 3 , wherein
 the exhaust gas purification device includes an NO X  storage reduction catalyst that is configured to store NO X  in the exhaust gas when an air-fuel ratio of the exhaust gas is a lean air-fuel ratio higher than the stoichiometric air-fuel ratio and to reduce NO X  stored in the NO X  storage reduction catalyst when the air-fuel ratio of the exhaust gas is a rich air-fuel ratio lower than the stoichiometric air-fuel ratio, 
 the supplier performs the in-cylinder rich control to reduce NO X  stored in the NO X  storage reduction catalyst, when a NO X  storage amount of the NO X  storage reduction catalyst becomes equal to or greater than a predetermined upper limit storage amount, and 
 the controller is programmed to predict that the in-cylinder rich control is performed in the predetermined time period when the NO X  storage amount of the NO X  storage reduction catalyst is equal to or greater than an allowable storage amount which is smaller than the upper limit storage amount, while predicting that the in-cylinder rich control is not performed the predetermined time period when the NO X  storage amount of the NO X  storage reduction catalyst is less than the allowable storage amount. 
 
     
     
       6. The failure diagnosis device of the emission control system according to  claim 4 , wherein
 the exhaust gas purification device includes an NO X  storage reduction catalyst that is configured to store NO X  in the exhaust gas when an air-fuel ratio of the exhaust gas is a lean air-fuel ratio higher than the stoichiometric air-fuel ratio and to reduce NO X  stored in the NO X  storage reduction catalyst when the air-fuel ratio of the exhaust gas is a rich air-fuel ratio lower than the stoichiometric air-fuel ratio, 
 the supplier performs the in-cylinder rich control to reduce NO X  stored in the NO X  storage reduction catalyst, when a NO X  storage amount of the NO X  storage reduction catalyst becomes equal to or greater than a predetermined upper limit storage amount, and 
 the controller predicts that the in-cylinder rich control is performed in the predetermined time period when the NO X  storage amount of the NO X  storage reduction catalyst is equal to or greater than an allowable storage amount which is smaller than the upper limit storage amount, while predicting that the in-cylinder rich control is not performed in the predetermined time period when the NO X  storage amount of the NO X  storage reduction catalyst is less than the allowable storage amount. 
 
     
     
       7. The failure diagnosis device of the emission control system according to  claim 3 , wherein
 the exhaust gas purification device includes an NO X  storage reduction catalyst that is configured to store NO X  in the exhaust gas when an air-fuel ratio of the exhaust gas is a lean air-fuel ratio higher than the stoichiometric air-fuel ratio and to reduce NO X  stored in the NO X  storage reduction catalyst when the air-fuel ratio of the exhaust gas is a rich air-fuel ratio lower than the stoichiometric air-fuel ratio, 
 the supplier performs the in-cylinder rich control to remove a sulfur component from the NO X  storage reduction catalyst when a sulfur poisoning amount of the NO X  storage reduction catalyst becomes equal to or greater than a predetermined upper limit poisoning amount, and 
 the controller predicts that the in-cylinder rich control is performed in the predetermined time period when the sulfur poisoning amount of the NO X  storage reduction catalyst is equal to or greater than an allowable poisoning amount which is smaller than the upper limit poisoning amount, while predicting that the in-cylinder rich control is not performed in the predetermined time period when the sulfur poisoning amount of the NO X  storage reduction catalyst is less than the allowable poisoning amount. 
 
     
     
       8. The failure diagnosis device of the emission control system according to  claim 4 , wherein
 the exhaust gas purification device includes an NO X  storage reduction catalyst that is configured to store NO X  in the exhaust gas when an air-fuel ratio of the exhaust gas is a lean air-fuel ratio higher than the stoichiometric air-fuel ratio and to reduce NO X  stored in the NO X  storage reduction catalyst when the air-fuel ratio of the exhaust gas is a rich air-fuel ratio lower than the stoichiometric ratio, 
 the supplier performs the in-cylinder rich control to remove a sulfur component from the NO X  storage reduction catalyst when a sulfur poisoning amount of the NO X  storage reduction catalyst becomes equal to or greater than a predetermined upper limit poisoning amount, and 
 the controller predicts that the in-cylinder rich control is performed in the predetermined time period when the sulfur poisoning amount of the NO X  storage reduction catalyst is equal to or greater than an allowable poisoning amount which is smaller than the upper limit poisoning amount, while predicting that the in-cylinder rich control is not performed in the predetermined time period when the sulfur poisoning amount of the NO X  storage reduction catalyst is less than the allowable poisoning amount. 
 
     
     
       9. The failure diagnosis device of the emission control system according to  claim 3 , wherein
 the exhaust gas purification device includes a selective catalytic reduction catalyst that is configured to adsorb NH 3  included in the exhaust gas and reduce NO X  in the exhaust gas using the adsorbed NH 3  as a reducing agent, and an NH 3  producing catalyst that is placed upstream of the selective catalytic reduction catalyst and is configured to produce NH 3  when an air-fuel ratio of the exhaust gas is a rich air-fuel ratio lower than the stoichiometric air-fuel ratio, 
 the supplier performs the in-cylinder rich control to produce NH 3  by the NH 3  producing catalyst when an NH 3  adsorption amount of the selective catalytic reduction catalyst becomes equal to or less than a predetermined lower limit adsorption amount, and 
 the controller predicts that the in-cylinder rich control is performed in the predetermined time period when the NH 3  adsorption amount of the selective catalytic reduction catalyst is equal to or less than an allowable adsorption amount which is larger than the lower limit adsorption amount, while predicting that the in-cylinder rich control is not performed in the predetermined time period when the NH 3  adsorption amount of the selective catalytic reduction catalyst is greater than the allowable adsorption amount. 
 
     
     
       10. The failure diagnosis device of the emission control system according to  claim 4 , wherein
 the exhaust gas purification device includes a selective catalytic reduction catalyst that is configured to adsorb NH 3  included in the exhaust gas and reduce NO X  in the exhaust gas using the adsorbed NH 3  as a reducing agent, and an NH 3  producing catalyst that is placed upstream of the selective catalytic reduction catalyst and is configured to produce NH 3  when tin air-fuel ratio of the exhaust gas is a rich air-fuel ratio lower than the stoichiometric air-fuel ratio, 
 the supplier performs the in-cylinder rich control to produce NH 3  by the NH 3  producing catalyst when an NH 3  adsorption amount of the selective catalytic reduction catalyst becomes equal to or less than a predetermined lower limit adsorption amount, and 
 the controller predicts that the in-cylinder rich control is performed in the predetermined time period when the NH 3  adsorption amount of the selective catalytic reduction catalyst is equal to or less than an allowable adsorption amount which is larger than the lower limit adsorption amount, while predicting that the in-cylinder rich control is not performed in the predetermined time period when the NH 3  adsorption amount of the selective catalytic reduction catalyst is greater than the allowable adsorption amount.